The seismic mass of that sensor is constructed in the form of a rocker which is both mechanically and electrically connected to the stationary substrate of the structural component. While the distribution of mass of the rocker is asymmetrical in relation to the torsion spring system, the rocker has two capacitor surfaces which are arranged and constructed symmetrically in relation to this spring system, and each of which forms a capacitor together with the substrate. An acceleration acting on the sensor structure causes the rocker to rotate or tilt around the spring system, thus causing a change in the difference of capacitances between these two capacitors. By evaluating the difference of capacitances or its change, it is possible to determine the acceleration acting on the sensor structure. The known acceleration sensor has a vertical sensitivity, so that accelerations perpendicular to the plane of the chip are detectable with the known sensor.
The moving parts of the sensor structure of the known sensor may only be deflected within certain limits without mechanical damage such as a broken spring or electrical short circuits occurring. Overload accelerations may also result in greater deflections of the moving parts, however, and thus to corresponding damage. For that reason, the known acceleration sensor is equipped with stops for the torsion springs. These stops are located in the area of connection between the torsion spring and the rocker and are rigidly connected to the substrate of the structural component, so that the deflection of the torsion springs and the motion of the rocker in the x/y plane, i.e. parallel to the plane of the chip, is limited. The geometry of the stops is not adapted to the expected deformation and deflection of the torsion springs, so that the stops and the torsion springs make contact only at points or edges in the event of a corresponding overload acceleration.
In regard to acceleration in the z direction two cases must be distinguished: acceleration into the substrate and acceleration out of the substrate. In the first of these cases the motion of the torsion spring and of the seismic mass is easily limited by a stop at an electrically neutral location on the substrate. In contrast, a motion of the seismic mass out of the substrate, as is to be expected in the second case, is not easily limited. Hence in the case of a structural height of around 10 μm, deflections of 10 μm or more may result in the seismic mass being elevated out of its surroundings, and consequently in the sensor structure getting stuck.
In non-directed drop tests of the known acceleration sensor, conchoidal breaks were found on the outer edge of the stops and on the torsion spring. Such conchoidal breaks may modify the mechanical properties of the spring system or result in development of cracks as incipient damage to the sensor structure. This may cause changes to the characteristics of the sensor such as sensitivity, offset and test signal. Furthermore, conchoidal breaks are a source of particles that may cause electrical short circuits or also mechanical blocking of the rocker. On the whole, the aforementioned conchoidal breaks usually result in quality-relevant degradation of the sensor function, and in extreme cases may even result in total failure of the sensor function.